The present invention relates to optical communication systems in general, and the link quality and link quality estimation between a transmitting station and a receiving station in particular.
For infrared device communication the Infrared Data Association (IrDA) has published a series of specifications designed to allow data communication between devices using the Infrared (Ir) medium. The paper of F. Gfeller and W. Hirt, xe2x80x9cRequest for Comments on Advanced Infrared (Alr) Physical Layer Specification (IrPHY)xe2x80x9d, presented at the IrDA Meeting in Toronto, Apr. 17, 1997, describes the concept of Robust Headers (RH) to support reliable collision avoidance with the Infrared Medium Access Control (IrMAC) protocol, which is responsible for coordinating the access to the infrared medium between infrared devices. The use of a RH in the physical layer header allows the coexistence of devices with different angular and range characteristics while maintaining the collision avoidance properties.
The International Patent Application PCT/IB96/00002 with publication number WO97/25788, filed on Jan. 3, 1996, describes an optical communication system enabling communication between several coexisting transmitting and receiving stations. In order to allow communication between coexisting stations, a robust physical layer header (herein referred to as Robust Header; RH) is employed which can be understood by all participating stations.
The two above-mentioned publications form the basis and the state of the art for the present invention and are incorporated by reference.
According to these references, infrared devices communicate in peer-to-peer mode where the transmitting station provides a Request-to-Send (RTS) frame to the receiving station to announce the transmission of a data packet, the receiving station provides a Clear-to-Send (CTS) frame in case of correct reception of the RTS frame, at least to the transmitting station, and the transmitting station subsequently sends the data frame to the receiving station. According to the general IrMAC frame format requirements, the RTS frame and the CTS frame comprise at least a Preamble field (PA), a Synchronization field (SYNC), and a Robust Header (RH) field. The PA field allows for reliable carrier sensing, symbol clock synchronization, and chip clock phase acquisition down to very low Signal-to-Noise Ratios (SNR). The PA field comprises symbols forming a periodic sequence of pulses, the number of slots per symbol (for example, L-slot Pulse Position Modulation; 4-slot (4-PPM) coding is proposed for AIrPHY) and the symbol content being known to all participating stations. The SYNC field, also a field with sequences of legal 4-PPM symbols, enables exact identification of the start of the RH field and consists of a certain predefined number of legal 4-PPM symbols. The RH field comprises according to the above referenced documents several fields of fixed length and known structure. By means of these fields, the receiving stations are informed about the signaling method used for data transmission. Further, the fields are used to provide other control information for the communication link or for the exchange of information to allow negotiation and/or adaptation of the data rate used for transmission in order to optimize the throughput depending on the quality of the channel.
The RTS frame further comprises, according to the above-mentioned IrDA Meeting paper, address fields called Source Address and Data Address (SA/DA) fields which follow the RH field. Further, this RH field is always robustly coded using repetition coding with a Rate Reduction of sixteen (RR=16) to provide maximum detection sensitivity. RR defines the level of repetition coding in order to ensure a correct data transmission; thus, every symbol is repeated RR times. The resulting redundancy in the symbol stream is intended to be exploited with suitable digital processing methods in the demodulator circuit and provide a SNR gain at the expense of a reduced data rate of 4/RR Mb/s. This is equivalent to individually matching the electrical receiver bandwidth to the reduced data rate. In contrast to this complex method the proposed method allows a virtual bandwidth reduction without having to physically change the receiver bandwidth and thus allowing a simple and optimal receiver implementation matched to the base rate of 4 Mb/s. The RR factor is defined to take on the values 1, 2, 4, 8, and 16, where RR=1 corresponds to the base rate of 4 Mb/s with 4-PPM and no repetition coding. The available data rates for the defined values of the RR factor are 4, 2, 1, 0.5, and 0.25 Mb/s. Every reduction step provides a SNR gain of nominally 3 dB electrical, corresponding to 1.5 dB gain in optical power. Thus, the nominal SNR gain increases from 0 dB to 12 dB electrical as RR changes from one to sixteen, respectively. Please note that for the remainder of this application relative power levels will be measured in the optical domain; a relative optical power level ofxc3x97dB corresponds to 2xc3x97dB relative electrical power or 2xc3x97dB relative SNR change. The common data rate of 0.25 Mb/s with RR=16 must be supported by all systems within a communications cell, or within a subcell of a communication cell. This data rate is used in the RH field of every transmitted frame and serves to convey information relevant for the MAC and PHY layers. With increasing data rate reduction the communication system is capable of operating under successively worse SNR conditions. This can be used to gain a larger transmission range up to twice the transmission range at 0.25 Mb/s compared to the base rate of 4 Mb/s, or to maintain the link quality under hostile channel conditions such as high levels of background light. Further, repetition coding reduces the detrimental effect of interfering signals and can serve as power management by using a higher data rate for shorter distances which minimizes the energy consumption per transmitted frame.
In contrast to the RTS frame the CTS frame ends with the RH field. The RH field of the CTS frame comprises a RR* field instead of a RR field which is similar to the RR field in the RTS frame, except that it specifies recommendations for the RR to be used in the reverse direction based on an evaluation of the link quality or some other indication. Thus, it is a recommendation by the destination device that the source should use this parameter when communicating with it.
According to the IrDA Meeting paper, data frames comprise after the PA, SYNC, and RH fields a main body field which comprises a large data field which is followed by a Cyclic Redundancy Check (CRC) field. The CRC field is a variable repetition coded 32 bit field and is used in well-known manner for checking whether the transmission has been successfully accomplished.
The frames according to the prior art are transmitted with a single fixed optical power and use a fixed RR in the header. This does not allow to estimate the link quality for 4 Mb/s data rate (which corresponds to RR=1) with short packets, for example, RTS and CTS frames. Further, it does not provide sufficient SNR margins for expected parity mismatch conditions. Please note that the concept of parity for Ir systems has been introduced in the above-referenced IrDA Meeting publication.
The general object of the invention therefore is to provide absolution for improved collision avoidance properties and link quality estimation between the participating stations within the Alr concept.
It is thus a further object of the invention to provide a method which ensures only a connection between the stations if a certain prescribed SNR for a chosen RR is met.
It is another object of the invention to provide solutions for the introduction of special frames for link quality analysis.
It is a further object of this invention to propose an energy saving mode.
The invention as claimed is intended to meet these objectives. It provides a method for wireless optical communication between a transmitting station and a receiving station and frames for use in wireless optical communication systems, whereby the transmitting station provides a RTS frame to the receiving station to announce the transmission of a data packet, the receiving station provides a CTS frame in case of correct reception of the RTS frame, and the transmitting station subsequently sends a data frame to the receiving station. Please note that the CTS frame is sent from the receiving station at least to the transmitting station which issued the RTS frame. Both frames (RTS and CTS) comprise the generally required PA, SYNC, and RH fields. The RTS frame further comprises at least a Source Address/Destination Address field (SA/DA) and a Cyclic Redundancy Check field (CRC). Adjusting the power level of the RTS frame and/or the CTS frame to be different than the nominal transmission power level at which data frames are sent and by variable repetition coding within the robust header parts (for example of the RTS frame), allows a larger dynamic range of link quality estimation and improved collision avoidance properties. Depending on the chosen optical power level, which can be higher or lower than the nominal transmission power level of the data transmission, and a variable repetition coding depending on the chosen optical power level, a number of different possibilities to improve the connection between the participating stations are proposed and claimed.
A higher power level of the CTS frame but without variable repetition coding (i.e. RR=16), for example, increases the transmission range of the CTS signal with respect to the data packet. Thus, hidden terminals can be better reached, collisions are more efficiently avoided.
According to a preferred embodiment, the RTS frame is transmitted with a constant transmission power level less than the nominal transmission power level and with a Rate Reduction RR=16 of the RH field and a variable RR=1, 2, 4, 8, or 16 of the SA/DA field and the following CRC field. The transmission power level is chosen such that in combination with the chosen RR of the SA/DA field a correct decoding of the SA/DA field by the receiving station is achieved on average for a desired SNR. Advantageously, the transmission power level is in the range of 50% to 75% of the nominal transmission power level.
According to another embodiment of the invention, the RH field in the RTS frame comprises a differentiating sub-field (discovery field; DISC), designating the RTS frame as a discovery frame, and a broadcast destination address in the SA/DA field. This allows a procedure with discovery frames using increasing values of RR which are sent by a transmitting station until it receives an answer from a receiving station.
In a further preferred embodiment the fields of the RTS frame and of the CTS frame are transmitted with a higher transmission power level than the nominal data transmission power level. Additionally, the RTS frame comprises a trailer with at least one Trailer field (TR) which follows the CRC field with a lower power level than the nominal power level. The power level and RR of said RH field of said RTS frame are correspondingly chosen to produce equal detection sensitivity as with RR=16 and nominal power level. Thus, the RTS field comprises at least two different power levels. One purpose of this approach is to reduce the total length of the RTS frame and another purpose is to increase the dynamic range of the link quality analysis by counting illegal 4-PPM symbols in the trailer section. It is possible to use RR=2, 4, or 8 with the corresponding higher power levels which provide equal detection sensitivity as with RR=16 and nominal power. This condition is fulfilled with increased optical power levels of 4.5 dB, 3 dB, or 1.5 dB relative to nominal optical power and use of RR equal to 2, 4, or 8, respectively. Preferably, the higher power level of the RTS/CTS fields is twice the nominal power and therefore RR=4. Further, in another embodiment the lower power level of at least one TR field is advantageously half the nominal power because this means a reduction of 3 dB in optical power which is a preferred value when using a variable optical power level changing with 1.5 dB steps.
For a fixed RRxe2x89xa016, for example RR=4, the pattern in said SYNC field is inverted in relation to the standard pattern to indicate to the receiving station that the RTS frame is sent with a different, and for the participating stations known, fixed RR (here RR=4).
Another preferred embodiment which can be used for evaluating the link quality makes use of a trailer which comprises several TR fields with different power levels, preferably the different optical power levels decrease in 1.5 dB steps starting with twice the nominal power level.
For reduced electrical power consumption when transmitting data at a very short range (app. 0.5 m distance) and increased data throughput for members (stations) with reduced optical power level, the optical power levels of the RTS, CTS, and data frames are transmitted according to a further embodiment with an optical power level less than the nominal power level and, instead of variable repetition coding within the robust header parts of said RTS/CTS frames, the SYNC field is inverted in relation to the standard pattern to indicate a reduced power level.